The present invention relates generally to mobile satellite antenna systems, and more particularly, to a top-fed variable pitch omnidirectional quadrifilar helix antenna system for use in mobile satellite applications.
It is highly desirable that antennas used in mobile satellite applications have an omnidirectional radiation pattern shaped to receive signals only at elevations of the satellite(s) with which they are employed. The satellites that typically operate with the mobile equipment are located at elevation angles above 25-30 degrees. The gain of the antenna system should be maximized above this lower elevation limit up to zenith. At the same time gain of the antenna system should be reduced below the horizon. Heretofore, no antennas have been available that meet all of these requirements.
The basic form of a resonant quadrifilar helix antenna was published in December 1970 in xe2x80x9cThe Microwave Journalxe2x80x9d. Since its initial development, research has been performed that vary the number of turns along with the length and diameter ratios. All of these factors affect the radiation pattern produced by the antenna. Conventional fractional turn design produces a cardioid radiation pattern. A tall narrow quadrifilar helix antenna exhibits a shaped-conical pattern with high grain to the horizon and decreased gain overhead, which is well suited to ground applications. Published data and designs regarding narrow antennas indicate that they are better suited to UHF applications.
It is therefore an objective of the present invention to provide for a top-fed variable pitch omnidirectional quadrifilar helix antenna system for use in mobile satellite applications.
The present invention provides for a top-fed quadrifilar helix antenna system which is wound with a special helical structure that changes pitch toward the top of the antenna. An exemplary top-fed quadrifilar helix antenna system includes first and second bifilar helical loops that each comprise a pair of orthogonal windings disposed in a mutual orthogonal relationship relative to a common central axis. Each loop is configured to have a winding pitch that varies along the cylindrical axis so as to suppress backlobe radiation from the antenna system.
First and second terminals are coupled to respective top ends of the bifilar helical loops. The first and second terminals may be fed using two sources, one for each pair of orthogonal windings of the first and second bifilar helical loops. This is preferable when the antenna system is used in satellite communication applications.
Each bifilar helical loop preferably comprises lower, intermediate, and upper sections whose pitch decreases from lower to upper. The antenna system may also include short circuits coupled to respective bottom ends the first and second bifilar helical loops.
The winding scheme significantly reduces the so-called xe2x80x9cbacklobexe2x80x9d radiation in the lower hemisphere. The backlobe angular region lies between xe2x88x9225 degrees and nadir. By reducing the backlobes, the antenna system may be placed and deployed on a wide range of vehicles, structures or mounting surfaces without suffering effects of scattering, reflections and coupling. These deleterious effects can greatly reduce the gain of the antenna in the upper hemisphere and cause problems with mobile satellite radio or communications equipment.
When using the present winding scheme, it is necessary to suppress both senses of circular polarization in the backlobe angular regions since circularly-polarized signals reverse their polarization sense when reflected from metal objects.
In mobile satellite applications, reflections from metal objects underneath the antenna system can act to cancel the desired direct signal from a satellite. This can greatly degrade a mobile satellite terminal""s performance.
Standard quadrifilar helix antenna systems have a backlobe that is oppositely sensed to the upper radiation. The present invention mitigates this effect.
The present quadrifilar helix antenna system was developed for use in a mobile satellite communications system. The quadrifilar helix antenna system receives digital signals from stationary or orbiting satellites in the 2.3 GHz frequency band.
The quadrifilar helix antenna system may be mounted to the exterior of many classes of vehicles including trucks, trains, cars, boats and other mobile or portable equipment. The quadrifilar helix antenna system may also be mounted to fixed structures.